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Offline tagging of radon-induced backgrounds in XENON1T and applicability to other liquid xenon detectors
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abstract
This paper details the first application of a software tagging algorithm to reduce radon-induced backgrounds in liquid noble element time projection chambers, such as XENON1T and XENONnT. The convection velocity field in XENON1T was mapped out using $^{222}\text{Rn}$ and $^{218}\text{Po}$ events, and the root-mean-square convection speed was measured to be $0.30 \pm 0.01$ cm/s. Given this velocity field, $^{214}\text{Pb}$ background events can be tagged when they are followed by $^{214}\text{Bi}$ and $^{214}\text{Po}$ decays, or preceded by $^{218}\text{Po}$ decays. This was achieved by evolving a point cloud in the direction of a measured convection velocity field, and searching for $^{214}\text{Bi}$ and $^{214}\text{Po}$ decays or $^{218}\text{Po}$ decays within a volume defined by the point cloud. In XENON1T, this tagging system achieved a $^{214}\text{Pb}$ background reduction of $6.2^{+0.4}_{-0.9}\%$ with an exposure loss of $1.8\pm 0.2 \%$, despite the timescales of convection being smaller than the relevant decay times. We show that the performance can be improved in XENONnT, and that the performance of such a software-tagging approach can be expected to be further improved in a diffusion-limited scenario. Finally, a similar method might be useful to tag the cosmogenic $^{137}\text{Xe}$ background, which is relevant to the search for neutrinoless double-beta decay.
Forward citations
Cited by 4 Pith papers
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Commissioning and first results from the Cold Radon Emanation Facility
Commissioning of CREF demonstrates a ~0.05 mBq radon assay capability at cryogenic temperatures and reports a preliminary factor-of-two suppression of 222Rn emanation in titanium.
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Flow-dependent tagging of $^{214}$Pb decays in the LZ dark matter detector
Flow-dependent tagging, built from paired radon-polonium decays, identifies 63% of 214Pb beta-decay backgrounds in LZ's fiducial volume at 9.0% exposure cost.
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Precise Measurement of $^{216}$Po Half-life with Exact Parent-daughter Pairing in PandaX-4T
Using exact space-time pairing of 220Rn-216Po decays in PandaX-4T, the 216Po half-life is measured to be 143.7±0.5(stat)±0.2(syst) ms.
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Radon Removal in XENONnT down to the Solar Neutrino Level
XENONnT reduced radon-222 in its liquid xenon target to 0.90 ± 0.07 micro-becquerel per kilogram, the lowest level reported for an operational dual-phase dark matter detector, matching the solar neutrino background.
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